Yeah, wheel diam is related to what you can easily roll over....rock crawlers know this.
Greater diam obstacle than about 25-30% of wheel diam gets near impossible to get over.
Bike wheels are NOT rated for any sideload at all. A few old aircraft replica guys have discovered
this to their great chagrin. Motorcycle wheels which were plenty strong for the weight were wiped
out with the first crosswind landing. Not much side load on your application, but good to keep
in mind. VERTICAL loads only, although genuine BMX wheels are way tougher than normal, and
probably are designed to get SOME small amount of offset loads, but still relatively little.
Only 30 year old, heck should be in mint condition.
Of course, at this point spending any money
on it would kinda 'break the spell'. You have really been getting this done for almost nothing, an
impressive feat in itself.
Bill